TaeguTec introduces inserts and holders to the RhinoRush family to optimise

Coolant. The issue of thermal stress leads some operators to believe that running the machine with coolant is the best option.

Shape. The experts agree that while different shapes of inserts may work, a round insert is the best option, followed by a square insert.

“If you have the corresponding toolholders in your facility already, a round insert is the strongest,” said Haid. “With a round insert, a rigid setup is necessary to take the impacts of the interrupted cuts. A square insert would be the second strongest. For the most part, diamond-shaped inserts are not well suited, but some interruptions could require a specific insert to get in and perform the cut.”

“Basically, you want to look for the strongest insert possible,” said Paul Lobsinger, sales and service engineer Canada, Greenleaf Corp. “It helps to have a tough negative insert to accept the interruptions and pounding.”

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Interrupted cutting in turning is subjective. While some operators may look at a part with a keyway and a small depth of cut as a heavy interruption, another operator may classify heavy interruptions only as multiple interruptions with a larger depth of cut. The workpiece material, part configuration and features, and depth of cut will all come into play when determining how the interruption will affect the overall process.

Ordering a machine with a Coromant Capto spindle is not a major obstacle, but the tool magazine and tool-change arm would have to be changed to handle Coromant Capto’s unique automatic-toolchanger gripper grooves, which are located on the flange. “It really falls on the machine tool builders,” Godfrey said. “If some day they see Coromant Capto as an option across the board in a shop, then they will be motivated to have that spindle option on all their machining centers. A shop that’s fully tooled with Coromant Capto has maximum flexibility and the best economy because they can minimize their assortment of tools.”

Tangential velocity on the surface of the tool or workpiece at the cutting interface. The formula for cutting speed (sfm) is tool diameter 5 0.26 5 spindle speed (rpm). The formula for feed per tooth (fpt) is table feed (ipm)/number of flutes/spindle speed (rpm). The formula for spindle speed (rpm) is cutting speed (sfm) 5 3.82/tool diameter. The formula for table feed (ipm) is feed per tooth (ftp) 5 number of tool flutes 5 spindle speed (rpm).

Alan holds a bachelor’s degree in journalism from Southern Illinois University Carbondale. Including his 20 years at CTE, Alan has more than 30 years of trade journalism experience.

Sandvik Coromant’s Coromant Capto machine adapted clamping units (MACU) for lathes are suitable for turning and milling applications.

“We were losing pressure with the Belleville washers, which is why we developed this unique gas-spring clamping mechanism,” said Sandvik Coromant’s Mills. He noted one customer who measured the clamping forces over a 2-year period on a Coromant Capto multitask machine and a machining center with a CAT steep-taper spindle found a significant loss of clamping force in the machining center and none in the multitask machine—even though both machines are the same age. “He’s a believer,” Mills said.

Rego-Fix offers monoblock Coromant Capto toolholders with ER and powRgrip collet systems. “The products were selected to offer the most popular Rego-Fix products for multitasking machines with a [Coromant] Capto interface,” said Peter Tester, president and CEO of Rego-Fix AG, Tenniken, Switzerland.

Ken Andersson, who has since retired from the toolmaker and now builds boats, invented Coromant Capto. Sandvik Coromant completed the first prototype in 1989 and unveiled the full version at IMTS 1990 in Chicago. Since then, Coromant Capto has made industry inroads via sales by Sandvik Coromant and its various licensees and was adopted in 2008 as an ISO standard (26623) for toolholding systems when the company’s patent expired and the need to license it ceased. ISO certification means no manufacturer can sell a Coromant Capto holder that doesn’t conform to the standard’s specifications.

“A simple example of this would be 50 per cent of the material taken away by voids or interruptions at the surface,” he said. “Fifty per cent of the surface remains in contact with the tool compared to an interrupted part. So, in this case, double the SFM to compensate for that.”

Those shares are for multitask machines with a Coromant Capto spindle interface as well as ones incorporating Coromant Capto as a modular tooling system using basic holders with Coromant Capto integration. A basic holder has a steep-taper or HSK design on the top and a female Coromant Capto clamping area on the bottom, and end users assemble cutting tools and extensions into the holder with a center screw. The basic holder stays in a machine and only the cutting tools have to be changed.

“For roughing applications, I would like to have a tougher grade with a larger T-land or a roughing chipbreaker with a nice edge prep to maintain the interruptions as well as hold that dimensional tolerance,” said Haid. “When finishing, go with something harder rather than something tougher to hold the tolerance and not wear out as fast.”

“However, with ceramics, we would recommend opting for a silicon-carbide, whisker-reinforced ceramic insert or a silicon nitride-based ceramic over an alumina-based or white ceramic, as it will provide much more strength and reliability,” said Goss.

“There needs to be a balance,” said Goss. “You don't want to have too much negativity or a large land on some of those soft, ductile materials, which could lead to excessive cutting force. But it also needs to be heavy enough to provide adequate performance and tool life.”

For example, a hard grade of carbide will run faster and last longer but is more brittle, whereas a tough grade of carbide has impact resistance but runs slower and does not last as long.

Haid added that it’s always better to start on the low end of the feed rate to provide a bit of wiggle room, rather than running a little bit slow. Running slow can put more impact on the insert.

Secures a cutting tool during a machining operation. Basic types include block, cartridge, chuck, collet, fixed, modular, quick-change and rotating.

In addition, Miller said on-machine spindle reconditioning, when needed, is not possible because of the complexity of the polygon and its 1:20 taper. “A [Coromant] Capto spindle needs to be pulled completely and rebuilt as a cartridge,” he said.

According to Sandvik Coromant, conventional tools, such as stick tools, represent about 87 percent of the global market for turning centers. Quick-change tools comprise the rest but are increasing in use. Within the quick-change category, Coromant Capto and the Kennametal KM system each have 43 percent shares in the U.S., according to Mills.

“When it comes to a multitask machine, Coromant Capto makes a ton of sense because you have a combination of rotating and static applications,” Godfrey said.

A key selling point for Coromant Capto tooling in the past was how fast its turning tools could be changed with Coromant Capto tooling compared to stick tools, but the current spotlight is on applying high-pressure coolant through the tool, according to Chris Mills, national portfolio manager for Sandvik Coromant Co. That’s because through-tool, high-pressure coolant is more prevalent now, providing the ability to improve chip control and reduce the temperature at the tool/workpiece interface. “You can increase tool life and cutting speed with high-pressure coolant,” he said.

Luminoso has a bachelor of arts from Carleton University, a bachelor of education from Ottawa University, and a graduate certificate in book, magazine, and digital publishing from Centennial College.

“Although positive inserts are generally not recommended for interrupted cuts, sometimes they are unavoidable,” said Mitchell. “In situations such as these, it is highly recommended to use a double-sided positive insert. Traditional positive inserts sit in a pocket and the only thing preventing them from moving is a small screw. But [some inserts] utilize a dovetail pocket, ensuring no movement even in interrupted applications, while still providing a positive cutting action.”

Machining operation in which material is removed from the workpiece by a powered abrasive wheel, stone, belt, paste, sheet, compound, slurry, etc. Takes various forms: surface grinding (creates flat and/or squared surfaces); cylindrical grinding (for external cylindrical and tapered shapes, fillets, undercuts, etc.); centerless grinding; chamfering; thread and form grinding; tool and cutter grinding; offhand grinding; lapping and polishing (grinding with extremely fine grits to create ultrasmooth surfaces); honing; and disc grinding.

In addition to the system’s high level of repeatability when mounting tools, Rego-Fix’s Weber pointed out that the full flange contact against the spindle that Coromant Capto provides means a tooling assembly’s Z-axis height is highly repeatable. He added that when machining at a spindle speed of 20,000 rpm or higher, a steep-taper spindle cavity opens a slight amount, pulling the toolholder deeper into the spindle and altering the Z-axis height. “Because a [Coromant] Capto holder sits against the spindle, you won’t get that,” Weber said. “You would get the same benefits with the BIG Plus and HSK, but [Coromant] Capto is an entire system versus the others, which are just individual holders.”

Coromant Capto tooling is plumbed to channel high-pressure coolant to reduce cutting temperatures and improve chip control.

Geometry. Another consideration for interrupted cutting is the geometry of the insert. Choosing an insert that has a small nose angle and long, narrow cutting edge does not offer the rigidity needed in most interrupted cutting applications.

1. Tooling system comprised of standardized tools and toolholders. 2. Devices that allow rapid mounting and replacement of tools. Commonly used with carousel toolchangers and other computerized machining operations. See toolchanger; toolholder.

With interrupted cuts there is a greater risk for the cutting edge to break down prematurely or inconsistently, lowering part quality. It is important to choose the best cutting tool and process parameters during interrupted turning operations. Phuchit/iStock/Getty Images Plus

“Tungaloy developed a technology that was able to draw the cobalt to the outer edges of the insert, giving just the cutting edge a higher concentration of cobalt,” said Mitchell. “This allowed for the best of both worlds: a carbide grade with a high concentration of tungsten to allow for higher cutting speeds and longer tool life, while at the same time providing additional concentration of cobalt at the cutting edge to provide additional impact resistance.”

“For interrupted-style cuts on hardened material, it’s best to choose heavier edge preparations,” said Goss. “With ceramic, having a large single or compound chamfer/land, or with carbide, having a heavier-duty chip form, will ensure the insert is able to handle the abusive nature of the cut and redirect those forces appropriately to allow for maximum tool life.”

Regardless of how a shop classifies interrupted cuts, interruptions can present themselves in a couple of different ways.

“The benefit of dealing with interrupted cuts in the roughing stage is that the insert can be changed, from part to part, without an offset,” said Matthew Haid, application specialist, Dormer Pramet, Mississauga, Ont. “Dealing with interrupted cuts in finish-turning can be much more challenging. If the part has a tight finishing tolerance, operators typically need to adjust the tool wear offset on the CNC over the course of the insert’s life. When changing the insert, it's recommended not to start at zero or the regular offset, but rather do a first pass to confirm the tolerance dimension. Both interrupted roughing and finishing can be a challenge, but with roughing, there is always material left to be finished, whereas finishing tends to be more sensitive.”

But the Coromant Capto coupling system isn’t only suitable for turning. “The main reason Coromant Capto was designed was to have a system that was equally at home for turning, milling and drilling,” Mills said.

A multitask machine’s main spindle has to lock into place for turning and free up for rotating applications, and Coromant Capto benefits those machines. “The tooling assumes that the B-axis, the actual milling head, will tip at 45° to approach the part,” explained Alan Miller, engineering manager for BIG Kaiser Precision Tooling Inc., Elk Grove Village, Ill. “With that, we can put the cutting tool on the centerline of the spindle, so there’s much less torque generated and the spindle doesn’t have to resist as much.” That extends spindle life by putting less pressure on the locking system that holds the spindle still when turning.

The Coromant Capto coupling was initially accepted in lathes because of its ability to enable quick tool changes—Sandvik Coromant says up to five to 10 times faster than conventional tools—with camshaft-style clamping. “It takes a quarter turn of a wrench to clamp and unclamp the cutting heads,” said Brent Godfrey, Sandvik Coromant’s product specialist for tooling systems. “When you set tools offline, for example, you can literally change a tool and be cutting again within 30 seconds.”

Even when new, the pull-stud or drawbar clamping force of a gas-spring mechanism is greater than either an HSK 100 or CAT 50 steep-taper holder’s disc-spring mechanism. According to Sandvik Coromant, the drawbar pressure for an HSK 100 holder is about 16,000 N, about 25,000 N for a CAT 50 holder and 45,000 N for a gas-spring C8 holder (see Table 1). “You can apply that much force to the shape without deforming it,” Mills said. “If you apply that much clamping force to the HSK, then the coupling is not going to be physically strong enough [to resist deformation.]”

Workpiece is held in a chuck, mounted on a face plate or secured between centers and rotated while a cutting tool, normally a single-point tool, is fed into it along its periphery or across its end or face. Takes the form of straight turning (cutting along the periphery of the workpiece); taper turning (creating a taper); step turning (turning different-size diameters on the same work); chamfering (beveling an edge or shoulder); facing (cutting on an end); turning threads (usually external but can be internal); roughing (high-volume metal removal); and finishing (final light cuts). Performed on lathes, turning centers, chucking machines, automatic screw machines and similar machines.

Some of the impacts of interrupted cuts on the turning process can, depending on the severity of the interruption, lead to accelerated tool wear. This increased tool wear can lead to more tool changes during the process, which ultimately leads to an increase in part cycle times and an overall increase in labour and tooling costs for the job. Here are some factors to consider to help limit tool wear.

Way of displaying real-world objects in a natural way by showing depth, height and width. This system uses the X, Y and Z axes.

“When you get a basic holder into a machining center, now you have a female Coromant Capto polygon that you can stick any type of tool into,” Sandvik Coromant’s Godfrey explained. “The bottom of a basic holder has a ground surface that’s the same diameter as the ground surface on the adapter it’s going to connect to. When you clamp the two tools together, those two surfaces come in contact with each other, and that achieves the face contact.”

When multitask, or mill/turn, machines started to become more popular around 2000, Coromant Capto made inroads into that machining segment. “Coromant Capto was probably designed a little bit before its time because it really was the multitask machine that demanded a coupling that was equally capable for turning, milling and drilling,” Mills said.

Speeds and Feeds. The experts agree that one of the biggest mistakes an operator can make during interrupted turning is to reduce speed.

When you are considering carbide grades, having either a large grain size or cobalt-enriched layer as part of the substrate will make for a stronger insert. Tungaloy

Levels of interrupted cuts can vary anywhere from a light interruption, which might account for only 10 to 20 per cent of the overall process, to a heavy interruption which can be 60 to 75 per cent or more of the overall process. Regardless of how you define interrupted cutting, there are some points to consider when undertaking an interrupted operation.

“When the patent ran out, we didn’t want people to make a lower-quality version so that it started to create problems for those who had Coromant Capto,” said Chris Mills, national portfolio manager for Sandvik Coromant, Fair Lawn, N.J. “We wanted to ensure that if anyone makes it, and anyone is more than welcome to make it, all the tolerances are exactly the same as the original. We drove this certification to make sure no one could change the design, and that’s what ISO certification achieves.”

Lindsay Luminoso, sr. editor/digital editor, contributes to both Canadian Metalworking and Canadian Fabricating & Welding. She worked as an associate editor/web editor, at Canadian Metalworking from 2014-2016 and was most recently an associate editor at Design Engineering.

BIG Kaiser Precision Tooling Inc.  (847) 228-7660 www.bigkaiser.com Rego-Fix Tool Corp. (317) 870-5959 www.rego-fix.com Sandvik Coromant Co. (800) 726-3845 www.coromant.sandvik.com/us

Without the need to perform multitasking, however, end users are more likely to gravitate toward standard steep-taper toolholders, which can be ground on conventional grinding machines. In contrast, tooling manufacturers need sophisticated CNC grinders to produce Coromant Capto’s polygon taper as well as a 3-D coordinate measuring machine to properly measure and qualify it, according to BIG Kaiser’s Miller. “Steep-taper tools are much easier to produce in large quantities and by more manufacturers, so the availability of tooling choices for steep taper is much larger,” he said. “It’s just a large expense to move to [Coromant] Capto for a traditional vertical or horizontal machining center.”

To grab, grab at, to seize. Those are definitions of the Latin word capto. For the Coromant Capto toolholding system developed by Sandvik Coromant Co., modularity, universality and repeatability define it.

Rego-Fix’s Tester concurred. “We find that the [Coromant] Capto system provides the best possible interface between the spindle and toolholder,” he said. “From the point of view of precision and performance, [Coromant] Capto comes very close to building the cutting edge directly into the spindle nose.”

Table 1: A comparison of drawbar pressure for rotating applications between Coromant Capto holders with a gas-spring clamping mechanism and CAT steep-taper holders and HSK holders.

Weber noted that was the case for the ER system when the Rego-Fix patent expired last decade and it became an open standard, DIN 6499.

Optimal strength is going to be realized by using a round geometry and a negative-style insert. Mitchell also noted that it’s important to pay attention to the chipbreaker selection. Some chipbreakers are designed for heavy interruptions, but others are designed to be free cutting and as sharp as possible, making them fragile and prone to chipping in an interrupted application.

Whether Coromant Capto penetrates the VMC and HMC markets remains to be seen, but the outlook for mill/turn applications is promising. “It will be the standard for multitasking machines,” Tester said. CTE

Choosing the right insert for interrupted turning can make all the difference in success of the overall process. One of the big misconceptions is that operators can just use whatever insert is available. Sometimes this may work out, but often operators will struggle.

1. Process of locating the center of a workpiece to be mounted on centers. 2. Process of mounting the workpiece concentric to the machine spindle. See centers.

Machines and machining/turning centers capable of performing a variety of tasks, including milling, drilling, grinding boring, turning and cutoff, usually in just one setup.

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Lobsinger explained that the smaller diameter value basically returns the operator back to the original recommended surface speed he is trying to achieve.

That flexibility comes from the Coromant Capto interface that allows interchangeability of tools for different machines, even if a shop purchased the tools on different occasions. The system’s modularity enables a tool to be built to the correct length based on component size. “The longest assembly I’ve seen has probably been 26 " or 27 " with the inclusion of a dampened Coromant Capto tool,” Godfrey said.

CNC machine tool capable of drilling, reaming, tapping, milling and boring. Normally comes with an automatic toolchanger. See automatic toolchanger.

The polygon taper ensures automatic radial centering and even pressure around the coupling, according to Sandvik Coromant, enabling repeatability within 2μm.

Choosing the right grade depends on the type and hardness of the material, as well as the level of interruption and the machining stage, whether roughing or finishing.

Mills estimated that about 8,000 machines with rotating spindles have Coromant Capto spindles and significantly more lathes incorporate the coupling. “I don’t have a good guess for that,” he said. “Maybe hundreds of thousands of machines more.”

Microprocessor-based controller dedicated to a machine tool that permits the creation or modification of parts. Programmed numerical control activates the machine’s servos and spindle drives and controls the various machining operations. See DNC, direct numerical control; NC, numerical control.

While Coromant Capto tooling is relatively expensive, with ISO certification the price for Coromant Capto holders should come down as the system continues to penetrate the market and is offered by more tooling manufacturers, according to Tester.

Fluid that reduces temperature buildup at the tool/workpiece interface during machining. Normally takes the form of a liquid such as soluble or chemical mixtures (semisynthetic, synthetic) but can be pressurized air or other gas. Because of water’s ability to absorb great quantities of heat, it is widely used as a coolant and vehicle for various cutting compounds, with the water-to-compound ratio varying with the machining task. See cutting fluid; semisynthetic cutting fluid; soluble-oil cutting fluid; synthetic cutting fluid.

About the Author: Alan Richter is editor of Cutting Tool Engineering, having joined the publication in 2000. Contact him at (847) 714-0175 or alanr@jwr.com.

A rigid setup is necessary for interrupted cuts to take the impact. A round insert is the strongest, followed by a square insert. Dormer Pramet

The Coromant Capto’s three-corner polygon shape distinguishes it from other toolholders. “Why does a modular [toolholder] system have to be circular?” Andersson asked. “I found the three-sided polygon provided superior stability.”

With carbide grades, having either a large grain size or cobalt-enriched layer as part of the substrate will make for a stronger insert. Larger carbide grains have more impact on strength compared to a fine-grade carbide.

Coromant Capto’s inroads into machining center applications are not as pronounced. Although Coromant Capto has a 30 percent global market share for modular tools in machining centers, according to Sandvik Coromant, the number of those machines with Coromant Capto spindles is small. “It’s just maybe a handful of shops that have dedicated machining centers shipped to them with Coromant Capto spindles,” Godfrey said.

Cone-shaped pins that support a workpiece by one or two ends during machining. The centers fit into holes drilled in the workpiece ends. Centers that turn with the workpiece are called “live” centers; those that do not are called “dead” centers.

In addition to having three sides, the hollow polygon has a 1:20, or 1.4°, taper. And with Coromant Capto, multiple versions are not an issue. “There is one version only,” Tester said, “not versions A to T, like HSK.”

Flexible-sided device that secures a tool or workpiece. Similar in function to a chuck, but can accommodate only a narrow size range. Typically provides greater gripping force and precision than a chuck. See chuck.

Coating. For interrupted turning, an insert with a physical vapour deposition (PVD) coating on the substrate will help extend tool life. PVD coatings are tougher than chemical vapour deposition (CVD) coatings.

“On machining centers, to be honest, the first choice is probably still a steep-taper holder,” Godfrey said, but noted that the introduction of the C10 size will be more attractive for machining center applications, which typically involve a lot of milling. Milling requires more torque and horsepower than static applications, and the new Coromant Capto size’s thicker and stronger polygon and 100mm of flange face contact increases bending stiffness. “On top of that, the amount of pull force on the coupling is going to be the highest of any size,” Godfrey said.

The impacts of interrupted cuts will depend on the process itself, whether it’s a roughing or finishing application. For the most part, roughing applications tend to include interruptions from forged or cast workpieces that are irregular in material or non-concentric shapes.

Coromant Capto spindles for rotating applications utilize a gas spring to actuate the clamping mechanism instead of the standard Belleville washer disc-spring mechanism. That’s done to provide a consistent maximum clamping force and longer component life because there are no mechanical parts to wear out. It also provides additional pull force, which increases bending stiffness, and enables better balance for high-speed applications because there are no moving parts.

For multitask machines, Coromant Capto has about a 40 percent global market share compared with KM’s 35 percent and 25 percent for HSK, BT and BIG Daishowa Seiki Co. Ltd.’s BIG Plus holders combined, according to Sandvik Coromant. For 2008 multitask machine tools sales, Mills estimated the shares as half for Coromant Capto, a quarter for BIG Plus, 15 percent for HSK and 10 percent for KM.

An interruption also can be experienced not in the physical sense of material discontinuity, but by an inconsistency in material hardness or having heterogeneity in the material makeup of the workpiece. Goss explained that heterogeneity in the material can occur because of precipitates or inclusions that form during the manufacturing process of the parent material. If these precipitates or inclusions are congregated in one area of the workpiece, they can also act as an interruption during the machining process.

The one version is currently available in six sizes: C3 (32mm-dia. flange), C4 (40mm), C5 (50mm), C6 (63mm), C8 (80mm) and C8x (a C8 polygon with a 100mm-dia. flange). A new C10 size, a 100mm-dia. flange with a larger polygon than the C8, will be introduced March 1.

“Most commonly, an interruption is a break in the continuity of the material during the machining process,” said Matt Goss, applications engineer and project development, Greenleaf Corp., Saegertown, Pa. “This is where the insert engages and disengages with the workpiece, either once or a series of times. These interruptions can either be induced as part of the machining process, such as turning over bolt circles or holes, slots, or keyways, or they can be inherent to the workpiece, for example if it is out of round or has forging scale on the surface. Each of these cases presents a scenario where there's a break in continuous and uniform flow of material across the cutting tool.”

“Availability and competition will certainly help the end user,” Miller said. “I would be a little cautious, though, as [Coromant] Capto is such a complex profile to grind that it is going to take some of these manufacturers a long time to get it right.”

Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.

“When it comes to ceramics, you want to avoid this temptation,” said Lobsinger. “It’s the first thing an operator will do when they start thinking something is going wrong, but you definitely don’t want to do that. The amount of increase in the recommended speed in severely interrupted cuts can usually be calculated. The operator can calculate the circumference of the part, then subtract the total sum of the interruptions. This will give a smaller diameter value, which increases the SFM. Essentially, when increasing cutting speed, you're reducing the time the insert spends out of the cut, thus reducing how much time the insert has to cool, ultimately reducing thermal stress and the growth of the cracks.”

The nozzle is nonadjustable, removing a variable from the process. “You’re not leaving it to the operator, who you really don’t want to decide where the coolant hits,” Mills said.

For interrupted cuts in softer or more ductile materials, having an edge prep with a larger or wider land is a good option. With ceramic, the insert does not need to be as negative as it would for a harder material; with carbide, a medium-duty chip form will provide additional shear, which is beneficial in most cases.

1. Ability of a material or part to resist elastic deflection. 2. The rate of stress with respect to strain; the greater the stress required to produce a given strain, the stiffer the material is said to be. See dynamic stiffness; static stiffness.

According to John Mitchell, general manager, Tungaloy Canada, Brantford, Ont., carbide cutting tools are made primarily of tungsten, a very hard, wear- resistant material that is bound together with cobalt. To produce a grade capable of handling interrupted cuts, traditionally cutting tool manufacturers simply used more cobalt. On the other hand, to make the insert more wear-resistant, capable of faster cutting speeds and longer tool life, manufacturers used grades that had less cobalt and more tungsten.

“However, coolant should almost always be shut off, unless machining an extremely gummy material,” said Mitchell. “When coolant is applied to an insert machining an interrupted cut, the insert undergoes extreme heat, then no heat when encountering the interruption. The hot insert is then suddenly submerged in coolant, which causes thermal cracking, shortening the tool life and potentially causing catastrophic failure.”

“PVD coatings have a thin layer that really holds up better against the impacts of interruptions versus the CVD option,” said Haid.

The polygon also provides torque transmission from the machine tool spindle to the cutting tool without the need for keyways. “A CAT or BT holder has keyways at the flange that transmits torque,” said Andreas S. Weber, president of Rego-Fix Tool Corp., Indianapolis. “Because it has a polygon shape, a [Coromant] Capto toolholder couldn’t turn inside a spindle, so the whole transferrable torque of the spindle is on the entire taper of the [Coromant] Capto holder.”

Mills noted that Coromant Capto comes plumbed to provide high-pressure coolant delivery, so there’s no concern about how to deliver the coolant to the nozzle. “With the nozzle, we very accurately hit at the right place on the insert with a controlled, precise jet,” he said, adding that 1,300-psi coolant delivery is typically standard on premium lathes and machining centers.

The experts noted that some operators tend to favour a positive geometry as they think it will help take the impact, but that is just not the case. A negative geometry is going to provide a much better cut in interrupted turning.